[GIS] Creating polygon layer from polyline layer in QGIS

geometry-conversionlinepolygon-creationqgis

Our systems expect polygon shapefiles, representing a collection of non-intersecting and geometrically valid (Multi)Polygons. Specifically, farm fields. We often receive our representations of farms and their fields as polyline shapefiles containing the field boundary polylines, typically with some small geocoding errors like dangling ends.

Is there a reliable way to transform the polyline layer into a polygon layer, where the perimeters of the polygons are calculated as minimal loops around polylines in the shapefile? I understand I'll probably have to tidy up the polylines first.

I am primarily interested in solutions involving QGIS or other free tools since my company doesn't buy any of the professional packages.

Best Answer

Converting lines to polygons will be easy: (Vector > Geometry Tools > Lines to polygons)

To deal with dangling ends, you could create a buffer around the polylines (Vector > Geoprocessing Tools > Buffer) and set them to 'dissolve'. This would attach any dangling endpoints to eachother.

Then convert the buffer polygons into lines (Vector > Geometry Tools > Polygons to lines)

Then convert the lines into polygons (Vector > Geometry Tools > Lines to polygons)

Related Solutions

[GIS] Creating polygons from points in Mapinfo

What you will need to do is write a MapBasic script something like the following.

Firstly, query your table. If you have co-ordinates stored as numbers in the table, the query will be something like this:

select x_column, y_column from yourTable [where condition] into MyQuery

Otherwise you need something like:

select CentroidX(obj), CentroidY(obj) from yourTable [where condition] into MyQuery

Then you need to iterate over the rows using Fetch statements. Then you can draw a polygon based on that point. The following is based on the Fetch() and Create Object() sections of the Mapinfo manual, and will probably have to be extensively altered to suit your case. In particular you may have to pay attention to co-ordinate systems if your data is not already projected in metres:

Fetch First From MyQuery
Do While Not EOT(MyQuery) 

                  Dim obj_region As Object 
Dim x(5), y(5) As Integer
Dim i As Integer 
x(1) = MyQuery.col1
y(1) = MyQuery.col2
x(2) = x(1) + 200
y(2) = y(1)
x(3) = x(1) + 200
y(3) = y(1) + 200
x(4) = x(1)
y(4) = y(1) + 200
x(5) = x(1)
y(5) = y(1)
' First, create an empty region object 
Create Region Into Variable obj_region 0 

' Now add nodes to populate the object: 
For i = 1 to 5 
Alter Object obj_region Node Add ( x(i), y(i) ) 
Next 

' Now store the object in the Sites table: 
Insert Into YourNewTable(Object) Values (obj_region)

    Fetch Next From MyQuery
Loop

[GIS] how to connect points to create a polygon boundary in QGIS

The plugin you are looking for is called Points2One. (You might have to allow experimental plugins to install it.)

Best Answer

Related Solutions

[GIS] Creating polygons from points in Mapinfo

[GIS] how to connect points to create a polygon boundary in QGIS

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